Method and apparatus for freezing aqueous liquid

ABSTRACT

A method of freezing aqueous liquids which method comprises the steps of: 
     a) introducing the aqueous liquid into a mould; 
     b) allowing at least the aqueous liquid in contact with said mould to freeze; and 
     c) releasing said frozen liquid from said mould; wherein said method further comprises the step of: 
     d) pre-cooling said mould to a temperature such that the frozen aqueous liquid can be readily released from said mould as a unitary structure.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for freezing aqueousliquids and, more particularly but not exclusively, is concerned with amethod and apparatus for freezing ice lollies.

BACKGROUND OF THE INVENTION

In the production of ice lollies a flavoured liquid is poured intomoulds each comprising several hundred cavities. The moulds are thenplaced inside a refrigerator and left until the flavoured liquid hasfrozen throughout. The moulds are then removed from the refrigerator andinverted. A stream of warm air is then blown onto the back of the mouldsto melt the outer surface of the lollies. As the surface melts thelollies slide downwardly out of their respective cavities.

It will be appreciated that the application of warm air requires heat tobe applied to the moulds which then has to be taken out when the mouldsare subsequently cooled. Furthermore, any sculptured features in thelollies become blurred and ill defined due to surface melting.

Various means have been tried to facilitate removal of the lollies fromtheir moulds without warming. Such means include coating the surface ofthe mould with a low friction coating, for example TEFLON (RTM), andcoating the mould with an organic release agent. These attempts havehowever been totally unsuccessful.

SUMMARY OF THE INVENTION

We have discovered that there is a direct correlation between the rateat which an aqueous solution is cooled and the propensity of the frozenliquid to adhere to a surface. In particular, as the rate at whichliquid is frozen is increased the tendency of the frozen liquid toadhere to a surface decreases. This was somewhat unexpected since in"The Friction and Lubrication of Solids", Part II, Chapter VIII, OxfordUniversity Press, Messrs. Bowden and Tabor observed that when a liquidwas frozen in contact with a block the adhesion between the frozenliquid and the block increased as the temperature decreased.

In the case of ice lollies, we have found that if the initial rate ofcooling is sufficient the moulds can simply be turned over and thefrozen lollies fall out (a small tap sometimes being necessary).

This has significant energy and throughput consequences. In particular,energy does not have to be wasted heating the moulds to release thelollies and then re-cooling the moulds. Furthermore, time does not haveto be wasted heating the moulds.

There is however a second consideration. In particular, if the initialrate of cooling is too high the lollies crack as they freeze and theproduct leaves the mould in several pieces.

It will be appreciated that the present invention is not limited to theproduction of ice lollies and is equally applicable to all aqueousproducts and, in particular, water containing foodstuffs includingchocolate ices. It is also applicable to the freezing of watercontaining pharmaceutical products.

WO/90 06693 discloses that the adhesion of water containing goods to aconveyor belt can be reduced by pre-cooling the conveyor before thewater containing goods are brought into contact with the conveyor.However, the disclosure does not specifically address the production ofice lollies and does not recognise that there is only a narrow band oftemperatures over which easy release can be obtained with lack ofstructural damage.

According to one aspect of the present invention there is provided amethod of freezing an aqueous liquid which method comprises the stepsof:

a) introducing the aqueous liquid into a mould;

b) allowing at least the aqueous liquid in contact with said mould tofreeze; and

c) releasing frozen aqueous liquid from said mould;

characterized in that said method further comprises the step of:

d) pre-cooling said mould to a temperature such that the frozen aqueousliquid can be readily released from said mould as a unitary structure.

Preferably, at least the surface of the mould intended to contact theaqueous liquid is pre-cooled to a temperature at or colder than -50° C.,preferably at or colder than -60° C., and advantageously between -70°and -80° C.

Advantageously, the surface of the mould intended to contact the aqueousliquid is NOT cooled below -85° C. and preferably not below -80° C. toinhibit structural damage to the frozen product.

Because of the necessity to warm prior art moulds it has been customaryto make the moulds of relatively thin material, for example aluminium orstainless steel with an average thickness of about 0.5 mm. In contrast,moulds for carrying out the present invention preferably have arelatively high thermal capacity to ensure sufficiently rapid cooling.Typically, the moulds will be made from aluminium or stainless steel andwill have an average wall thickness of at least 1 mm, and preferably atleast 2 mm.

If desired, the moulds may contain passageways for the passage of acoolant therethrough.

The aqueous liquid can simply be flavoured water or may comprise aviscous liquid, for example chocolate.

The aqueous liquid may be allowed to remain in the mould until frozenthroughout. However, in the preparation of certain delicacies a chargeof viscous chocolate may be poured into a mould, the surface of thechocolate frozen and the mould carefully inverted to allow the unfrozenaqueous liquid to flow from the mould. A different liquid or pastyproduct may then be poured into the crust and the whole allowed tofreeze throughout before inversion of the mould and release of thecontents with a sharp tap on the mould. It should be noted that carefultemperature control is necessary in this process to ensure that there isa little adhesion between the chocolate and the moulds so that thecrusts do not fall out of their cavities during the first inversion.

The present invention also provides an apparatus for carrying out amethod in accordance with the invention, which apparatus comprises amould made from thermally conductive material and having an averagethickness of at least 1 mm.

Reference is made to "average thickness" as moulds with fins designed tocreate features in the surface of the ice lollies are known. However,the distribution of such fins in moulds known to the inventors issparse.

Preferably, said average thickness is at least 2 mm.

Advantageously, said thermally conductive material is selected from thegroup comprising stainless steel, copper, aluminium, brass and mixturesthereof.

Preferably, said mould has at least one passage to allow a cryogen topass therethrough.

In one embodiment, said apparatus comprises a vessel, a mould having aplurality of cavities dividing said vessel into an upper portion and alower portion, means to admit a coolant into said lower portion, meansto withdraw coolant from said lower portion, means for filling saidcavities with ice lolly solution, means for moving said cavities, meansfor inserting sticks into said cavities downstream of said fillingmeans, and means for withdrawing frozen ice lollies from said cavities,and means for cooling said coolant with liquid nitrogen.

If desired, said cavities may, in use, be cooled by partial immersion insaid coolant. Alternatively, means may be provided for spraying coolantagainst said cavities.

In a particularly preferred embodiment, the coolant is only used forpart of the manufacturing cycle, the balance of the refrigeration beingprovided by a mechanical refrigeration unit. In such an embodiment thecoolant might be used for pre-cooling the moulds and, optionally, duringone or more of the following steps:

1. filling the cavities, and

2. initial cooling after filling.

Preferably, said apparatus includes means for introducing nitrogen fromsaid cooling means into the upper portion of said vessel.

Advantageously, said means for moving said cavities comprises a steppingmotor.

In another embodiment, said apparatus comprises a tunnel, a mould in theform of a continuous belt mounted in said tunnel, said mould having aplurality of cavities therein, means to admit a coolant into saidtunnel, means for filling said cavities with ice lolly solution, meansfor moving said cavities and means for inserting sticks into saidcavities downstream of said filling means.

For a better understanding of the invention reference will now be madeto the following Examples and the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of our test apparatus;

FIG. 2 is a graph showing the effect of precooling on adhesion;

FIG. 3 is a schematic vertical cross-section through one embodiment ofan apparatus in accordance with the invention;

FIG. 4 is a schematic vertical cross-section through a second embodimentof an apparatus in accordance with the invitation, and

FIG. 5 is a schematic plan view of a third embodiment of an apparatus inaccordance with the invention.

EXAMPLE 1

A multi-cavity mould of stainless steel approximately 2 mm in thicknessand at room temperature was filled with a solution of 10% (by volume)gelatin in water. The mould was placed in a mechanical refrigerator at-20° C. and left for 4 hours to freeze throughout.

On inversion of the mould the lollies remained fast in the mould andcould not be removed even when the mould was tapped with a small hammer.

EXAMPLE 2

The procedure of Example 1 was repeated but using solid carbon dioxideat -50° C. as the refrigerant. Again the lollies could not be extractedfrom the mould.

EXAMPLE 3

The procedure of Example 1 was repeated except the mould was cooled bybeing lowered into a bath of liquid nitrogen at -196° C. On invertingthe mould the lollies remained in place. They also remained in placeeven when the mould was tapped vigorously.

EXAMPLE 4

The procedure of Example 2 was repeated except that the mould waspre-cooled to -50° C. prior to adding the liquid. The lollies did notfall out of the moulds on inversion but did fall out when the mould wastapped firmly.

EXAMPLE 5

The procedure of Example 1 was repeated except that the mould waspre-cooled to -70° C. by exposure to vapour from liquid nitrogen. Oninverting the mould some of the lollies dropped straight out and theremainder fell out when the mould was tapped.

EXAMPLE 6

The procedure of Example 5 was repeated except the mould was replaced bya mould of stainless steel having a wall thickness of 1 mm. On invertingthe mould most of the lollies dropped straight out and the remainderfollowed on gently tapping the back of the mould.

From the above Examples we deduced that the rate at which heat isextracted from the liquid is critical. It will be noted that althoughthe mould in Example 3 was plunged in liquid nitrogen (-196° C.) thecooling raze was inadequate.

Clearly, from Examples 1 to 3 the cooling rate was too low whilst inExample 4 to 6 the cooling rate was sufficient.

To test our hypothesis the apparatus shown FIG. 1 was constructed. Theapparatus, which is generally identified by reference numeral 1comprised a brass block 2 weighing approximately 1.5 kg; a tube 3 ofplastics material 12 mm in diameter; a spring balance 5; a tension wire6 and a rod 7 which projected through holes in the tube 3 as shown,

In use, brass block 2 was placed in a refrigerator set to a particulartemperature for several hours until the brass block 2 was at a uniformtemperature throughout, The brass block 2 was then removed from therefrigerator and the tube 3, complete with the rod 7 and tension wireplaced thereon.

5 ml of an aqueous solution of 10% gelatin (by volume) was then pouredinto the tube 3, The arrangement was then returned to the refrigeratorand left for several hours for the solution to freeze throughout.

On removal from the refrigerator the tension wire was connected to thespring balance 5 which was raised slowly until the frozen solutionseparated from the brass block 2, The maximum reading of the springbalance 5 was recorded,

The procedure was repeated several times at various temperatures and theadhesive force (the reading on the spring balance 5 less the weight ofthe lifted mass) recorded,

The results are shown in FIG. 2,

It will be noted that although there is a spread in the results,particularly at higher temperatures, the adhesive force reduced verysubstantially as the temperature of the brass block 1 was lowered.

When the brass block 1 was pre-cooled to -70° C. no adhesive force couldbe measured with the spring balance 5.

In view of the work of Bowden and Tabor we concluded that lowtemperature itself is not the determining factor but the RATE ofcooling. It will be appreciated that in the experiments described abovethe 1.5 kg mass of the brass block 2 was sufficient to provide anessentially constant temperature source for freezing.

The calculated cooling rate in the liquid 1 mm from the interfacebetween the liquid and the mould has been indicated in FIG. 2. It willbe seen that at a cooling rate of greater than 650° C./min the adhesionis minimal. Transferring the above data into the commercial freezing ofice lollies it should be recalled that when the moulds are inverted theice lollies have a certain mass. However, and far more significantly, ifthe moulds are pivotally mounted and are allowed to drop against a barwhen the mould is inverted the impact will release the ice lollies eventhough they adhere to some extent to the moulds.

Quite acceptable cooling rates 1 mm beneath the surface would be of theorder of 650° to 750° C./min.

As indicated above the cooling rate need only be sustained sufficientlylong to ensure the formation of a frozen crust and the internal freezingcan proceed at a more leisurely pace if desired.

Obviously, the rate of cooling sustained must be sufficient so that theheat remaining in the liquid does not melt the frozen crust. The heattransfer rate to be sustained depends on the thermal conductivity of theliquid, for example an aerated ice cream has far lower thermalconductivity than a sugar solution for making ice lollies.

An interesting advantage of the present invention is that whereas theshape and configuration of features on the surface of lollies removedfrom conventional warm air released moulds is generally blurred, thefeatures on the surface of lollies frozen in accordance with theinvention is extremely sharp. In this connection, ice lollies andchocolate ices in the shape of animals, birds and figures are becomingincreasingly popular. Such shapes are made using moulds having partswhich can be physically separated to release the frozen product.

The present invention envisages a mould which, unlike prior art moulds,which are relatively thin, is relatively thick and has channels thereinfor the passage of a cryogen therethrough. However, for high productionrates it may be preferable to either partially submerge the moulds in aliquid cryogen or spray them with liquid cryogen.

Whilst we would strongly recommend liquid nitrogen as the source ofpre-cooling the moulds other cryogens could also be used. Solid carbondioxide might commercially be used although the majority of normalrefrigerants would not lower the temperature of the moulds sufficientlyto be effective in a commercial environment without the use of complexenergy expensive refrigeration systems.

Whilst Examples 1 to 6 clearly show the importance of pre-cooling themoulds to facilitate release, further work has shown that if the mouldsare excessively precooled then the frozen product is liable to sufferstructural damage.

EXAMPLE 7

A mould of stainless steel approximately 2 mm in thickness was placed ina liquid nitrogen refrigerator set to -90° C. overnight so that it had auniform temperature of -90° C. The mould was intended for making rocketshaped ice lollies having a cylindrical body approximately 10 cm longand 12 mm in diameter.

A solution of 10% (by volume) gelatin in water was introduced into themould. The water rapidly froze. However, during freezing there was asharp audible crack. When the mould was inverted about two-thirds of thebody fell out followed by the top one-third as a separate and distinctpiece. The product was thus unmarketable.

EXAMPLE 8

The procedure of Example 7 was repeated several times except the mouldwas pre-cooled to -85° C. The results at this temperature were varied.Seven samples broke in two with a muted crack. Five samples appearedquite satisfactory.

EXAMPLE 9

The procedure of Example 7 was repeated several times except the mouldwas pre-cooled to -80° C. All the ice lollies produced were found to bestructurally sound.

From the results of Examples 7 to 9 it is clear that excessively rapidcooling is undesirable. It is anticipated that the lowest temperature towhich a mould can be pre-cooled may vary a little according to thecomposition of the aqueous product.

In the commercial production of ice lollies using one know machine, themoulds are partially filled with liquid at a filling station. The sticksare then lowered into the cavities and held in position until the icelollies are frozen throughout. The moulds are then warmed and the icelollies lifted out of their cavities by their sticks when the mouldshave been warmed sufficiently.

When adapting this production technique for the present invention wehave noted that when the sticks are inserted into the aqueous liquidpart of the liquid is displaced and comes into contact with the wall ofthe cavity.

It is particularly important to ensure that the mould has sufficientthermal conductivity to ensure that this displaced liquid issufficiently rapidly cooled when it comes into contact with the wall ofthe cavity that it does not materially adhere thereto.

During the manufacture of ice lollies, the aqueous liquid is typicallypre-cooled to close to its freezing point prior to being introduced intothe moulds. The whole conventional process from introduction of theaqueous liquid to withdrawal of the frozen ice lollies typically takesabout 4 minutes. In contrast, the present invention typically reducesthis time by nearly one half.

It will be noted that accurate temperature control is important to thesuccess of the present invention and it is recommended that the mouldsthemselves be cooled by a liquid such as DOWTHERM (RTM), methanol orSYLTHERM XLT (RTM) whose waxing temperatures are -73° C., -85° C. and-110° C. respectively. The liquid itself should be cooled by direct orindirect heat exchange with liquid nitrogen.

Referring now to FIG. 3 there is shown an apparatus for the continuousproduction of ice lollies. The apparatus, which is generally identifiedby reference numeral 101 comprises a vessel 102 which is divided into anupper portion 103 and a lower portion 104 by a mould 105 containing amultiplicity of cavities 106.

The mould 105 is fabricated from stainless steel and is generally 2 mmin thickness. When viewed from above the mould 105 is of circularcross-section and can be rotated relative to the vessel 102 by means ofa stepping motor 107.

In use, liquid nitrogen is admitted to a heat exchanger 108 via acontrol valve 109. The liquid nitrogen cools a stream of coolant(SYLTHERM XLT (RTM)) from pipe 110 to -82° C. The cold coolant leavesthe heat exchanger 108 through pipe 111 and is admitted to the lowerportion 104 of the vessel 102.

It will be noted that the lower portion 104 of the vessel 102 is ofsubstantial volume and this provides a substantially constant lowtemperature source for cooling the mould 105. The coolant leaves thelower portion 104 of the vessel 102 through pipe 112 and is fed to theinlet of pump 113 which continually circulates coolant through thevessel 102.

The temperature of the coolant in pipes 111 and 112 is measured bytemperature sensors 114 and 115 respectively and the control valve 109is opened and closed as a function of the temperature sensed by thetemperature sensor 115 and the difference between temperature sensor 115and temperature sensor 114.

In use, the apparatus 101 is first cooled by the coolant.

Ice lolly solution is then pumped from chiller 116 by gear pump 117 to avalve 118. When valve 118 is closed the solution passes through pressurerelief valve 119 to the chiller 116.

Stepping motor 107 is activated at regular intervals to rotate the mould105 by a fixed amount. As each set of cavities 106 stops beneathdispenser assembly 120 valve 118 is opened for a fixed period of timeduring which the cavities 106 are filled to 1 cm from the top.

The ice lolly solution immediately starts to freeze and by the time thecavities 106 have been advanced six times by the stepping motor 107 theice lolly solution has formed a crust adjacent the cavities 106 althoughthe ice lolly solution in the centre of the cavities 106 is stillslushy.

At this time sticks are inserted into the cavities 106 at a stickinsertion station (not shown). The sticks penetrate the slushy liquidnear the bottom of the cavities 106 and, by the time the sticks arereleased before the next stepping operation are held securely in place.

After a further ten stepping operations the sticks are gripped and thewhole frozen ice lollies withdrawn upwardly from the cavities 106.

The cavities 106 are then re-cooled to their original temperature duringthe next four stepping operations after which the process is repeated.

The entire process from injection to removal takes 100 seconds.

In order to inhibit ice forming inside the vessel gaseous nitrogen fromthe heat exchanger 108 is passed through pipe 121 into the upper portion103 of the vessel 102 and then vented to atmosphere via fan 122 and vent123.

Whilst the cavities in FIG. 3 are cooled by immersion in coolant itwould equally be possible to spray the cavities with sufficient coolant.

Turning now to FIG. 4, there is shown an apparatus which is generallysimilar to the apparatus shown in FIG. 3 except that the lower portionof the apparatus is divided into two zones, one of which 103' usescoolant indirectly cooled by liquid nitrogen and the other of which 103"uses coolant indirectly cooled by a mechanical refrigeration unit (notshown). The zones 103' and 103" are separated by walls 125, 126 andcooling is effected by spraying the coolant in each zone against thecavities as they pass through the zones.

Briefly, as the cavities pass clockwise over wall 125 they arepre-cooled to approximately -82° C. The cavities are then filled withpre-chilled ice lolly solution at filling station 127. The filledcavities are then further cooled by liquid nitrogen cooled coolant untilthey pass over wall 126. At this point they are cooled by sprays of thesame coolant indirectly cooled by a mechanical refrigeration system to asomewhat warmer temperature. Sticks are inserted at stick insertionstation 128 and after substantially complete cooling the frozen icelollies are removed at removal station 129.

By using liquid nitrogen for providing the lower pre-cooling temperaturerequired and mechanical refrigeration for providing the in-depthfreezing the overall process becomes extremely attractive.

Referring now to FIG. 5, there is shown a tunnel freezer which isgenerally identified by the reference numeral 201.

The tunnel freezer comprises a tunnel 202 having a belt 205 providedwith a multiplicity of cavities 206.

In use liquid nitrogen is introduced into the tunnel 202 via valve 209and spray header 210 to maintain the interior of the tunnel 202 atapproximately -84° C.

The cavities 206a are thus at approximately -84° C. as they pass underdispenser assembly 220 which substantially fills the cavities with icelolly solution.

As the cavities 206 move along the tunnel 202 in the direction of arrow`A` the liquid immediately adjacent the cavities 206 forms a crustwhilst the liquid inside becomes slushy.

At this point a stick inserter 224 inserts sticks into the slush.Freezing is then completed as the cavities pass out of the end of thetunnel 202 where they simply fall out of the cavities as they pass overdrive wheel 225.

The tunnel 202 is provided with an extract fan 222 and vent 223. Thetunnel 202 is also provided with mixing fans 226 to induce turbulence inthe tunnel 202 and thereby enhance heat transfer.

Whilst the apparatus shown in FIG. 3 is intended for producing highquality ice lollies which can be sculptured to some extent the apparatusshown in FIG. 4 is intended for the inexpensive production of simple icelollies.

In the Examples disclosed the temperatures of the cavities were measuredusing a thermocouple on the inner surface of the cavity, i.e., thesurface which the aqueous liquid contacts.

What is claimed is:
 1. A method of freezing an aqueous liquid whichmethod comprises the steps of:positioning a mold fabricated from a metalused in freezing of foodstuffs and having a wall thickness of at least 1mm to receive the aqueous liquid; pre-cooling the surface of the moldintended to be contacted by the aqueous liquid to a temperature between-50° C. and -85° C.; introducing the aqueous liquid into the mold;allowing at least the aqueous liquid in contact with the mold to freeze;and releasing said frozen aqueous liquid from the mold.
 2. A methodaccording to claim 1, wherein said mould is pre-cooled to a temperatureat or colder than -60° C.
 3. A method according to claim 1, wherein saidmould is not pre-cooled to a temperature lower than -80° C.
 4. A methodaccording to claim 3, wherein said mould is pre-cooled to a temperatureof from -70° C. to -80° C.
 5. A method according to claim 1, wherein atleast part of said mould has an average wall thickness of at least 2 mm.6. A method as claimed in claim 1, wherein said aqueous liquid containssugar.
 7. A method according to claim 1, wherein said aqueous liquid isviscous.
 8. A method as claimed in claim 7, wherein said aqueous liquidis liquid chocolate.
 9. A method according to claim 1, wherein only theaqueous liquid adjacent the mould is allowed to freeze to form a crustand that said method includes the step of extracting (unfrozen) liquidfrom said moulds so that said crust forms a shell.
 10. A methodaccording to claim 1, wherein said cooling rate in said liquid 1 mm fromthe interface between said liquid and said mould is at least 650° C./minat least during initial freezing.
 11. A method according to claim 10,wherein said cooling rate is at least 750° C./min.
 12. A methodaccording to claim 1, wherein the rate of heat transfer subsequent tothe formation of an initial frozen crust is maintained at a levelsufficient to prevent the thawing of said crust.
 13. A method accordingto claim 1, wherein said mould is inverted to release said frozen liquidfrom said mould.
 14. A method according to claim 13, wherein said mouldis jarred to facilitate the release of said frozen liquid.
 15. A methodaccording to claim 14, wherein said mould is dropped against a bar as itis inverted to facilitate the release of said frozen liquid.
 16. Amethod according to claim 1, including the step of using mechanicalrefrigeration to help freeze said aqueous liquid after introduction intosaid mould.